As one of microphones which have conventionally been used in an information communicating terminal such as a portable telephone, an electret condenser microphone (ECM) using an organic film can be taken. The ECM is a microphone for disposing an electret causing a polymer material to have an electric charge on one of electrodes of a condenser and converting a change in an electrostatic capacity fluctuating corresponding to a sound pressure into a voltage change.
As one of characteristics of the microphone, a directionality can be taken. For example, a “non-directionality” (an omnidirectionality) having no directionality, a “unidirectionality” which easily catches a sound in a specific direction, an “ultradirectionality” which causes a directionality to have a narrower angle, and a “bidirectionality” which greatly catches a sound source in two forward and backward directions. The microphone is designed to have a specific directionality depending on uses.
FIG. 12A is a view showing a sectional structure of an ECM having a unidirectionality. As shown in FIG. 12A, in a microphone 100, a diaphragm 11 such as a metal conductor, a fixed electrode 12 having an electret film 13 formed thereon, and a printed circuit board 18 having a circuit element mounted thereon are disposed in a case 17 having a first sound hole 15A. An interval between the diaphragm 11 and the fixed electrode 12 is held by a spacer 14, and a back air chamber 16 is formed between the fixed electrode 12 and the printed circuit board 18. Furthermore, the case 17 has a second sound hole 15B formed on an opposite side to the first sound hole 15A. The diaphragm 11 is obtained by deposing aluminum on a film, for example.
The electret film 13 is a substance which is generally charged permanently (charge held) irrespective of an external electric field, and FEP (Fluorinated Ethylene Propylene fluorinated ethylene propylene resin) to be a fluororesin is used.
In the microphone 100, when the diaphragm 11 is vibrated at a sound pressure, an electrostatic capacity of a plate capacitor constituted by the diaphragm 11 and the fixed electrode 12 is changed and converted into a voltage change, and the voltage change is output from the microphone 100 through an amplifying circuit.
More specifically, a sound made on the second sound hole 15B side first enters through the sound hole 15B and reaches a back side of the diaphragm 11 (an indirect sound). The same sound turns around and also reaches a surface side of the diaphragm 11 with a slight delay (a direct sound). An obstacle (an acoustic resisting member) is put from the second sound hole 15B to the back side of the diaphragm 11 so that the indirect sound is delayed to arrive in phase with the direct sound. Consequently, the sound is offset as the same amount of energy generated on the surface and the back of the diaphragm 11 at the same time and is not electrically output.
On the other hand, a sound made on the first sound hole 15A side is first transmitted to the surface side of the diaphragm 11. Referring to the subsequent turn toward the back side, the arrival is further delayed due to the delay caused by the turn and the delay caused by the obstacle. Based on the time difference, the energy is not offset but is output electrically. Accordingly, the microphone 100 has a single forward directionality.
Thus, the ECM serves to convert the change in the electrostatic capacity fluctuating corresponding to the sound pressure into the voltage change. By providing a hole on a case, it is possible to design a directionality. In recent years, however, a reduction in a mounting cost has been demanded with a further reduction in a size and a thickness in the ECM. In the conventional ECM, an electret material to be an organic material which is less resistant to heat is used as described above. Therefore, the ECM cannot deal with surface mounting through a solder reflow and is to be attached to a substrate through a connector provided in the ECM so that a high cost is required for a connector component.
Therefore, there has been proposed a small-sized microphone (MEMS (Micro Electro Mechanical Systems) microphone) using a micromachining technique for practically using a semiconductor technology. FIG. 12B shows a sectional structure of the MEMS microphone.
As shown in FIG. 12B, an MEMS microphone 200 has a diaphragm electrode 23 and an electret film 24 formed on a silicon substrate 21 through a first insulating layer 22. A fixed electrode 26 provided with a sound hole 27 is formed thereon through a second insulating layer 25. Moreover, a back air chamber 28 is formed on a back face of the diaphragm electrode 23 by etching of the silicon substrate 21.
The diaphragm electrode 23 is formed of conductive polysilicon and the electret film 24 is formed by a silicon nitride film or a silicon oxide film. Moreover, the fixed electrode 26 is formed by laminating the conductive polysilicon and the silicon oxide film or the silicon nitride film.
In the MEMS microphone 200, when the diaphragm electrode 23 is vibrated at a sound pressure, an electrostatic capacity of a plate capacitor constituted by the diaphragm electrode 23 and the fixed electrode 26 is changed and is fetched as a voltage change.
In the MEMS microphone 200, thus, an electret material to be an inorganic material is used. Therefore, it is possible to implement reflow mounting which cannot be carried out in a conventional ECM, to decrease the number of components and to reduce a size and a thickness (see Patent Document 1).
Patent Document 1: JP-A-2001-245186 Publication